While the present invention is useful generally wherever uniform extrusion of an extrudate onto a substrate is desired, the invention will be discussed primarily with regard to extruding tubular jackets about cables such as those used in the telecommunications industry.
In the manufacture of telecommunications cable a protective plastic jacket is often extruded over a stranded cable core. This is generally done with an extruder of the type having a crosshead. A crosshead serves to redirect and to reconfigure a stream of plastic fluid, usually cylindrically or rod shaped, into a tubular shape about the cable. This reconfiguration is generally performed with the use of a diverter tube that is seated within a cylindrical cavity of a crosshead block. The interior surface of the diverter tube is sized to receive the cable which is passed linearly therethrough, or to receive a core tube through which the cable is passed, while its generally cylindrical exterior surface is provided with raised lands that define channels. These channels are shaped so as to redirect the flow of plastic fluid introduced into the crosshead some 90.degree. and to divide it into two or more streams that are routed to a plurality of diverter tube channel passages spaced radially along the cylindrical cavity. From here the configuration of the raised lands is such as to permit the plastic fluid to spread into a tubular confluent that is drawn down upon the cable core just outside the extruder die.
As the fluidic plastic material must ordinarily follow a flow path having changes in both direction and path size, varying flow and balance conditions are inherently created. These flow and balances, wherein various portions of the flow at any one point along the path travel at different speeds, create circumferential variations in the thickness of the wall of the tubular confluent once it has solidified into a jacket. Since some minimum wall thickness is required for proper cable performance, these circumferential variations in jacket thickness must be compensated for by an increase in the average wall thickness. This, of course, increases manufacturing costs.
The just described problem of wall thickness variations in extruded, tubular jackets has heretofor been recognized and attempts made at providing solutions. These solutions have taken the form of crosshead designs that divide the stream of plastic fluid delivered to the crosshead into several smaller branch streams that are routed circumferentially about the cable and then recombined into a tubular confluent stream to equalize the flow rate of plastic circumferentially onto the cable. This has been done on a volumetric or flow rate basis, neglecting pressure and velocity distributions in the plastic stream. Though such designs have improved concentricity and roundness of tubular extrusions, they have not been satisfactory when the plastic utilized has been of a highly viscous or elastic type. Molten polypropylene or medium density polyethylene, for example, when moving through a conduit is subjected to shear stresses that result in substantial velocity and pressure gradients, particularly in channel bends and enlargements which do not readily return to steady state fluid flow conditions.
Two examples of the just described approaches are shown in U.S. Pat. Nos. 3,579,731 and 3,860,686. These patents disclose a crosshead having a compensation or diverter tube formed with an annular restriction located downstream of a fluid delivery port. The restriction has an axial length that tapers from an axially long surface located radially adjacent the delivery port to an axially short surface located radially opposite the delivery port. This construction has been found to perform well where the taper is designed for specific flow rate of a plastic fluid of known viscosity. Its effectiveness, however, is diminished significally when plastic fluids of other viscosities are used or where other flow rates are employed. Consequently, these approaches are not readily adaptable to changes in processing material, tooling and extrusion conditions. Furthermore, such prior art jacketing diverter tubes are difficult to fabricate due to their design complexity which leads to increased fabrication costs. In addition, use has been limited to the application of extruded insulation about wires as opposed to cables. It would be quite difficult to use this prior art apparatus in the manufacture of cables due to the size limitation which flow and pressure requirements impose on cable jacketing extruders.
Another problem resulting from existing cable jacketing diverter tubes is the precise alignment which must be maintained between the diverter tube and the incoming plastic flow stream. Also, weld lines formed on the tubular jacket are often weak.
More recently, an improved diverter tube has been described in U.S. Pat. No. 4,279,851. Here, a stream of plastic fluid is forced into the crosshead and the stream is bifurcated into two branch stream. Each branch stream is channeled into two diametrically opposed locations about the cable where each stream is shaped with the selected prespread width. Each stream is spread from each of these locations into a confluent stream about the cable of tubular shape having a circumference of between 22 and 50 times the prespread width of each branch stream at an axial to lateral spread ratio of between 1:2 tangent 30.degree. and 1:2 tangent 40.degree..
While this latter design is less complex and less costly than the former mentioned apparatus, there still exists the problem of the formation of weak weld lines and the lack of adaptability to rheological changes due to changes in processing material, tooling and extrusion conditions as well as a desire for further diverter cost reduction.
It can therefore be seen that need still remains for the development of practical and cost effective methods and means for extruding plastic jackets of uniform tubular wall thickness about cables and the like, especially, for methods and means which are adaptable to changes in processing materials, tooling and extrusion conditions.